This invention relates to helical antennas wherein the diameter of the helix is short compared with the wavelength of the radiated signal. The helix is very long compared with its diameter, and substitutes for a wire, a pipe, or a tower as a radiating element. The directivity pattern is essentially the same in any case. The radiating element, to be resonant, must have an "electrical" length that is an appreciable fraction of a wavelength of the radiated signal, generally 1/4 to 1/2 the wavelength.
Any straight radiating element has an electrical length only slightly longer than its physical length. A helix has an electrical length much longer than its physical length. The helical radiating element can thus be considerably shorter than an equivalent straight radiating element. This feature is particularly important when the frequency of the radiated signal is below 50 mHz and radiating elements are necessarily long, requiring considerably space and costly supporting structure.
This invention further relates to antennas wherein the electrical length is adjusted to be resonant at the frequency of the radiated signal. Antennas of this type are called self-resonant antennas, and are distinguishable from fixed-length antennas wherein the system must include an auxiliary coupling device to match the antenna to the rest of the system.
Finally this invention relates to antennas that are continuously variable in electrical length. Continuously variable antennas are distinguishable from antennas that are tapped with several switchable electrical lengths. U.S. Pat. No. 3,179,941 is typical of the switched-length, helical antenna art.
Prior art has used helices that are continuously variable in electrical length U.S. Pat. No. 2,948,894 and U.S. Pat. No. 3,623,113). However, the helices of prior art have fixed dimensions, are wound around dielectric forms, and are thus not collapsible. At minimum frequency the fixed-dimension helix radiates over its entire length. At higher frequencies, portions of the helix are shorted-out with a straight element, reducing the electrical length of the shorted-out portion to the electrical length of the shorting element. It is apparent that a fixed-dimension helix has a resonant frequency range limited approximately to the ratio of the electrical length of the helix to the physical length of the helix.